A conventional apparatus for measuring the frequency response in an optical receiving system comprises a light source from which an intensity-modulated light radiates and means demodulating the light to be received from the light source. In operation, the frequency response of a photodetector in an optical receiving system is measured in accordance with the demodulation of the intensity-modulated light.
Another conventional apparatus for measuring the frequency response in an optical receiving system is described in "Measurement of High Frequency Response of Long Wavelength Optical Detector using Optical Heterodyne Detection" by Satoki Kawanishi et al. on page 376 of "National Conference Record, 1985, Semiconductor Devices and Materials, the Institute of Electronics and Communication Engineers for Japan." The apparatus of measuring the frequency response in an optical receiving system comprises two semiconductor lasers from which laser lights having the wavelength of, for instance, 1.2886 .mu.m and 1.2887 .mu.m radiate, a half mirror combining the laser lights respectively focussed by lenses, a single mode optical fiber propagating the combined light, a photodetector receiving the light output from the optical fiber to produce an electrically converted beat signal, a spectrum analyzer measuring output power of the beat signal in regard to frequency thereof, and a peltier element cooling and heating one of the two semiconductor laser to control the beat frequency.
According to the apparatus for measuring the frequency response in an optical receiving system, the oscillation frequency of the semiconductor laser varies by 20 GHz with temperature change by 1.degree. C., so that the frequency sweep can be performed by the temperature control thereof while the output light of the semiconductor laser is maintained constant. For this reason, the measurement of the frequency response can be performed with a higher precision.
In the former apparatus for measuring the frequency response in an optical receiving system, however, a disadvantage results in that the precise measurement of the frequency response is not achieved at the high frequency band of more than GHz since when a single laser is directly modulated, the intensity modulation is included in the frequency response characteristic in the measuring result of the photodetector.
In the latter apparatus for measuring the frequency response in an optical receiving system, there further results the disadvantages that the cost is increased for the reason that two semiconductor lasers having a single oscillation frequency must be comprised therein, and that the construction thereof is complicated for the reason that the temperature control of the semiconductor laser must be performed, for instance, by the peltier element.